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Experimental Transmission of African Horse"Sickness by Means of Mosquitoes ( * )

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Experimental Transmission of African Horse Arch. lost. Razi, 1966, 18, 119 - 125 EXPERIMENTAL TRANSMISSION OF AFRICAN HORSE"SICKNESS BY MEANS OF MOSQUITOES ( * ) Y. Ozawa. G. Nakata, and F. Shad-del SUMMARY African horse-sickness was transmitted by means of the bites of Anopheles stephensi aTld Culex pipieTls which had engorged infected horse blood 15 to 22 days previously. Febrile response (40.4 to 40.5 C.) occurred approximately 16 days after the 2 horses used were bitten by infected mosquitoes. Death followed in 9 to 12 days. The cause of death was confirmed as African horse-sickness by postmortem findings and laboratory tests. Introduction It has long been known that African horse-sickness is not directly contagious. Many species of arthropods have been suspected as vectors of the disease because horse-sickness is usually most prevalent during warm wet seasons and can be pre­ vented by insect control. As early as 1912, Schuberg and Kuhn 9 reported that they had transmitted horse-sickness mechan;cally by me ans of Stomoxys clllcitrans. Williams 11 reported Lypprosia minuta as a possible transmitter. Van Saceghem 10 suspected ticks and members of the order Diptera. Carpano 1 incriminated the dipterous genera. Anoph­ eles, Aedes, Phlebotomus, and S'imulillm as possible carriers of African horse-sick­ ness virus. The assertions of the latter 3 authors have been made without support­ ing experiment. Nieschulz et 111.,5 reported that Aedes mosquitoes were not significant vectors, as they could keep horse-sickness virus for only 1 week after experimental infec- ':' Reprint from the American Journal of Veterinary Research, Vol. 26, No. 112, 744-748 From the Near East Animal Health Institute, a cooperative United Nations Special Fund-FAO Project with the Ministry of Agriculture, Razi Serum and Vaccine Institute, Teheran, Iran. 119 tion. Furthermore extensive investigations by Nieschultz et 111.6 on mosquitos as potential victors of horse-sickness virus turned out to ,be negative, and they con­ cluded that mosquitoes were not vectors of horse-sickness. Du Toit 6 recovered African horse-sickness virus by inoculating horses with a suspension of Culicoidl's caught in the wild state. lt has not been determined how long ~ulicoides may carry the virus or whether Cl/timides may biologically transmit the 'disease trom horse to horse. Since this report, various species of Culimides have been widely believed to be the major vectors of African horse-sickness as sum­ marized in the recent report by Maurer and McCully.3 In the present experiment, ATlophel"s s(epheTlsi Liston and one of the most com­ mon mosquitoes, Culex pipiens, were used for transmission experiments and horse­ sickriess was biologically transmiUed to horses. Materials and Methods Mosquitoes.-Cull'x pipiens were caught in Teheran in April, 1963, and succes­ sive breedings were made at the inse:.:tarium of the Ne3r East Animal Health In­ stitute, Razi Institute. The 16th generation was used in the experiment. Anopheles s(ephl'Tlsi were also caught in Iran' The colony * had been main­ tained at the insectarium of the Institute of Parasitology and Malariology, Teheran University. Eight additional successive breedings were made before use in the ex­ periment. Horses.--Nonvaccinated horses, approximately 12 months old, were obtained from an isolated village in the mountainous area by the Caspian Sea. They were treated. with appropriate anthelmintics and kept under close observation for 1 week before' the experiment commenced. Virus.-Horse 20 was infected with African horse-sickness virus by intraven­ ously injecting the 5th mouse passage of Iranian strain 10/60. 9 The inoculum was 6,300,000 LD50 in suckling mice. Blood obtained from the horse 1 day before it died of horse-sickness (2 wk. after the injection of virus) was defibrinated and stored ,at 4 C. This blood was used as the material for infecting mosquitoes. Virus Isolation~Demonstration of virus from the blood of infected horses and from the tissues of dead horses was carried out by injecting the materials into the q~ains of 5-week-old mice. Defibrinated blood specimens were diluted with an ~, 'Pa.rtof the colony obtained through courtesy of Dr. Modlfl, Institute of Parasi­ tology, and Malariology, Teheran University, Teheran, Iran. 120 equal volume of saline solution. Tissues from infected horses :vere mi.xed with an equal volume of saline solution and ground in Ten Broeck tIssue gnnders. The tissue suspension was centrifuged at 2,000 r.p.m. for \0 minutes, and the super­ natant fluids were used as inoculum. Each specimen was injected into 10 mice, e3ch given 0.03 ml. of inoculum. The brains of infected mice in extremis were har­ vested and stored at -25 C. Neutralization Tests-Antibody levels in blood collected from infected and con­ trol horses were determined before and after infection. All the collected serums were heated at 56 C. for 30 minutes and the antibody titers measured by the standard virus dilution method described previously * * The same techniques were employed for identification of the virus isolated, using mice instead of tissue culture in tubes. Transmission Experiml'Tll.-The defibrinated blood of infected horse 20 was obtained on Feb. 25, 1964 (13 days postinoculation). Female mosquitoes of each of of the 2 species were allotted to 2 groups, A and B; the 4 groups of mosquitoes were kept in separate cages and fed with in­ fected blood on different days (Table 1). Before being fed the blood, al! the mos- TABLE 1-Timetable of Successful TransmiSSion Experiments Incubation No. of pcriod of nl06quitocs Date ferl with virus in enJ;orged/ l\.lo~quito inCccted Dute Cod on mosquitocs* No. plaeed Horse ,g-roup hlood normal hOTSf"S (days) on horses No. A 1HJ}J/I eles .del'hetlsi (A) :\[a1"('h 1-~ Mareh 18-19 15-18 14/30 TE-3 .11Jo}Jhele ..~ stephensi (13) Mareil 1-3 ('1I'eI pi/,iens (A) Fen. 26-27 Mareh 18-19 15-22 2/19 TE-6 Cft/ex pipiens (B) lI!arch 2-:1 * ~Iinimum and maximum inlervnls bclwcen Ihe lst mosQuito fecding with infected horse blood and tho 2nù fec(ling on the normal horses. quitoes were starved for more th an 8 hours by removing the water and 5% glucose wlution from the cages. Then, group A of each species was fed with undiluted in­ fected blood and group B with the blood diluted in an equal volume of physiologie saline solution. To feed the mosquitoes, cotton wool placed in small petri dishes was saturated by adding 12 ml. of either the diluted (B) or undiluted (A) infected horse blood. One petri dish was placed in each cage overnight for about 16 hours, after which the engorged mosquitoes were transferred to separate cages. ,•• , Hazrati, A., and Ozawa, Y.; Serological Studies of Afrlcan Horse-Slckness Virus-With Emphasis on Neutralization Test in Tissue Culture: Unpublishec;1 data, 1964, 121 These engorged mosquitoes were kept alive by feeding every day with fresh water and 5 % glucose solution. They were also fed calf serum twice a week. The temperature range of the insectarium was 23.0 to 26.5 c., and the range of relative humidity was 62 to 78%. Ali horses were maintained in insect-proof rooms throughout the experiment. At the end of an incubation period of 15 to 22 days, mosquitoes of group A and B of the same species were pooled and placed on a healthy horse by the fol­ lowing method: The mosquitoes were placed in a cage, 15.0 by 8.5 by 7.3 cm., made with a strong metal frame and fabric (organdy). The bottom of the cage was closely attached to the shaved skin of the horse's back. This small cage was covered with another stronger cage of metal with wire mesh. These cages were tightly fixed to horses with belts and springs for about 16 hours. During this period, the cages were covered with thick cio th to keep out the cold. Two additional horses were placed in the same stable during the experiment. One of them was actually exposed to infected mosquitoes, C. piPÙ'IIS, but none of the mosquitoes engorged the horse's blood. The other horse was kept without any treatment. Body temperatures of all horses were recorded twice daily throughout the ex­ periment Results African horse-sickness virus was in the defibrinated blood of horse 20 used to feed mosquitoes. The virus was identified as the Asia type virus by serum neutra­ lization tests. Virus was also found in the blood of hor,es TE-6, collected wh en they had revers above 40 C. and identified as the same Asia type of Afri:an horse-sickness virus by neutralization tests in mice. The course of the disease in infected horses was fol\owe:! by recording their body temperatures and any abnonnal signs. Horses 20, TE-6, and TE-3 had similar temDerature responses (Fig. 1), except that the temperature rise appeared severer in the ino:.:ulated hors 20 than in those horses infected by mosquito bites. AIso, the disease was much more protracted in the horses in­ fected by mosquito bites, and they lost a con,iderab'e amount of weight during the course of the di,ease. During the last 2 days be­ ,. fore death, these 3 horses could not 5J remain standing. The other 2 DA vs AFTER E XPOSURE horses, TE-I and TE-5, which were Fig. l-Postexposure temperatures <c.) of horses not incculated and on which no 20. TE-6, and TE-3. 122 mosquito engorged blood, did not have signs of disease, and their body tempera­ tures remained between 37 and 39 C. throughout the experiment.
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